1. Field of the Invention
The present invention relates to an image reader using CCD line sensors, such as a prepress flat-bed color scanner, and more particularly to the technique of reducing noise components included in image signals output from the CCD line sensors.
2. Description of the Prior Art
Output voltage of a CCD line sensor is not 0 V even when there is no incident light (i.e. in a light-shielded state). In order to read an image or the like through a CCD line sensor without impairing its tones, therefore, it is necessary to set an output voltage in the light-shielded state for use as a reference (hereinafter referred to as black reference level) and to subtract this black reference level from an output voltage occurring during a light reception to obtain an output voltage proportional to a quantity of incident light.
To obtain the black reference level noted above, a CCD line sensor normally includes, at ends of its pixel array, a plurality of pixels which are constantly in a light-shielded condition (hereinafter referred to as black reference pixels). Conventionally, compensation for the output of the CCD line sensor is carried out using the black reference pixels as set out below.
The output signal of one particular black reference pixel or an average of output signals of a plurality of black reference pixels in the CCD line sensor is sampled and held as the black reference level for one line. The black reference level is subtracted from output signals (image signals) of effective pixels, thereby to compensate for the output of the CCD line sensor.
However, the known construction noted above has the following disadvantage:
According to the known construction, the output compensation is effected by subtracting the same black reference level from all image signals covering one line read by the CCD line sensor. If noise is included in the black reference level, all the image signals for one line are prone to its influence. The image recorded by means of these signals will reflect the noise in the form of a streak often striking to sight.
With the method of using an average of output signals of a plurality of black reference pixels, the noises included in the individual black reference pixels are leveled out. This, of course, results in a reduction in the influence of noise compared with the method of using the output signal of one black reference pixel as the black reference level. However, an image scanner such as a prepress color scanner carries out logarithmic transformation of image signals to obtain density signals. Since the scale of shadow portions is enlarged through this operation, the noise still is visible in the form of a streak in the shadow portions.
Through the researches conducted to overcome such a disadvantage, the inventors have found that, apart from the noise being enlarged in the shadow portions as a result of the logarithmic transformation, noises in color difference signals also are enlarged when color correction is effected for the color difference signals in a color changing unit. This fact will be described hereinafter with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram showing an outline of a conventional image reader using CCD line sensors. FIG. 2 schematically shows levels of B, G and R components in various parts of the image reader and noise levels superposed on the respective components.
In FIG. 1, references 1B, 1G and 1R denote CCD line sensors for detecting blue (B), green (G) and red (red) color components of light transmitted from a color original. Output signals from the CCD line sensors 1B, 1G and 1R are converted by an analog-to-digital converter 2 into digital image signals DB, DG and DR, respectively, which undergo an output compensation process at an output compensator 3 using a black reference level as described hereinbefore. The image signals after the output compensation undergo a logarithmic transformation at a logarithmic transformer 4, and are then applied to a color change circuit 5. FIG. 2 (a) shows the image signals B, G and R as applied to the color change circuit 5. As illustrated, noises .+-.NB, .+-.NG and .+-.NR are superimposed on the respective image signals.
The color change circuit 5 functions to change these log-transformed color image signals B, G and R into four color components of yellow (Y), magenta (M), cyan (C) and black (K). The image signals B, G and R output from the logarithmic transformer 4 are applied to a color difference signal extractor 51 and a luminance signal detector 52. The luminance signal detector 52 detects, as a luminance signal Max, the signal having the highest signal level among the image signals B, G and R input thereto. It is assumed here that the image signal B is detected as the luminance signal Max (see FIG. 2 (b)).
The color difference signal extractor 51 extracts color difference signals CB, CG and CR by subtracting the input image signals B, G and R from the luminance signal Max, respectively. As a result of this extraction process, as shown in FIG. 2 (c), noises .+-.NB and .-+.NG are superimposed on the color difference signal CG, and noises .+-.NB and .-+.NR on the color difference signal CR. The color difference signal CB is in zero level at this stage.
The color difference signals CB, CG and CR are applied to a basic color corrector 53 for correcting turbidity of color inks and variations in spectral characteristics of the optical system. The color corrector 53 includes three lookup tables corresponding to the color difference signals CB, CG and CR. Each lookup table is formed of a memory which receives the corresponding color difference signal as an address, and outputs data stored in a corresponding address region as a color-corrected image signal.
FIG. 3 shows a typical example of color correction characteristics of the color corrector 53. Such color correction characteristics are set to the individual lookup tables. In FIG. 3, references KB, KG and KR are coefficients for multiplying the input color difference signals CB, CG and CR to effect color correction, i.e. enlargement ratios of the color difference signals, respectively. In FIG. 3, these coefficients correspond to the gradient of the color correction characteristics. As seen from FIG. 3, an ordinary image reader has such color correction characteristics that, in order to provide a good color reproduction in highlight portions, the gradient is made steep in a region of small color difference signals to enlarge the small color difference signals. Thus, it will be understood from FIG. 3 that the noises input to the color corrector 53 as superimposed on the color difference signals are enlarged in the region of small color difference signals.
FIG. 2 (d) shows color difference signals CY(KB.multidot.CB), CMKG.multidot.CG) and CC(KR.multidot.CR) corrected by the color corrector 53. As seen, noise components expressed by (.+-.NB.-+.NG)KG are superimposed on the color difference signal CM, and noise components expressed by (.+-.NB.-+.NR)KR on the color difference signal CC. The color difference signals corrected in this way are applied to an adder 55 in the next stage.
On the other hand, the luminance signal Max output from the luminance signal detector 52 is applied to a subtracter 54 as well as the color difference signal extractor 51 noted above. The subtracter 54 forms a black signal K by subtracting the luminance signal Max from a white reference signal W shown in FIG. 2 (e). As shown in FIG. 2 (f), a noise .-+.NB is superimposed on the black signal K. This black signal K is applied to the adder 55. The adder 55 outputs image signals Y, M and C resulting from color changes effected by adding the black signal K to the corrected color difference signals CY, CM and CC, respectively.
As shown in FIG. 2 (g), a noises expressed by .-+.NB is superimposed on the image signal Y, noises .+-.NB(KG-1).-+.NG.multidot.KG on the image signal M, and noises .+-.(KR-1).-+.N.multidot.KR on the image signal C. The noise (.+-.NB.-+.NG)KG superimposed on the image signal M results from the noise .+-.NB.-+.NG superimposed on the color difference signal CG and multiplied by KG at the color corrector 53. The noises (.+-.NB.-+.NR)KR superimposed on the image signal C result from the noises .+-.NB.-+.NR superimposed on the color difference signal CR and multiplied by KR at the color corrector 53.
As described with reference to FIG. 3, the noises included in the color difference signals are enlarged where the color difference signals are small. Moreover, maximum values of B, G and R are reduced in a shadow portion, with the color difference signals tending to be small. As a consequence, the noises become emphasized in the shadow portion.
It will be appreciated that the noises superimposed on the image signals are generated in the shadow portions, and that the more readily are the noises generated the smaller, i.e. the closer to gray, the color difference signals are. This fact identifies with the way in which noises appear on an actual recorded image.